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SOI1 encodes a novel, conserved protein that promotes TGN-endosomal cycling of Kex2p and other membrane proteins by modulating the function of two TGN localization signals.

Brickner JH, Fuller RS - J. Cell Biol. (1997)

Bottom Line: Deletion of SOI1 improved retention of Tyr713Ala Kex2p in the pro-alpha-factor processing compartment but, unlike the original soi1 alleles, did not increase the half-life of Tyr713Ala Kex2p.This hypothesis was confirmed in several ways.TLS2 delayed exit of Kex2p from the TGN, whereas TLS1 did not affect this step.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor 48109-0606, USA.

ABSTRACT
Localization of yeast Kex2 protease to the TGN requires a signal (TLS1) in its cytosolic tail (C-tail). Mutation of TLS1 results in rapid transit of Kex2p to the vacuole. Isolation of suppressors of the Tyr713Ala mutation in TLS1 previously identified three SOI genes. SOI1, cloned by complementation of a sporulation defect, encodes a novel, hydrophilic 3,144-residue protein with homologues in Caenorhabditis elegans, Drosophila melanogaster, and humans. Epitope-tagged Soi1p existed in a detergent-insensitive, sedimentable form. Deletion of SOI1 impaired TGN localization of wild-type Kex2p and a fusion protein containing the C-tail of Ste13p, and also caused missorting of carboxypeptidase Y and accelerated vacuolar degradation of the Vps10p sorting receptor. Deletion of SOI1 improved retention of Tyr713Ala Kex2p in the pro-alpha-factor processing compartment but, unlike the original soi1 alleles, did not increase the half-life of Tyr713Ala Kex2p. These results suggested that Soi1p functions at two steps in the cycling of Kex2p and other proteins between the TGN and prevacuolar compartment (PVC). This hypothesis was confirmed in several ways. Soi1p was shown to be required for optimal function of TLS1. Suppression of the Tyr713Ala mutation by mutation of SOI1 was shown to be caused by activation of a second signal (TLS2) in the Kex2p C-tail. TLS2 delayed exit of Kex2p from the TGN, whereas TLS1 did not affect this step. We propose that Soi1p promotes cycling of TGN membrane proteins between the TGN and PVC by antagonizing a TGN retention signal (TLS2) and facilitating the function of a retrieval signal (TLS1) that acts at the PVC.

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Model for role of  Soi1p, TLS1, and TLS2 in the  cycling of Kex2p between the  TGN and PVC. TLS2 inhibits/delays entrance of Kex2p  into the PVC transport vesicle at the TGN. Soi1p inhibits this function of TLS2.  TLS1 and Soi1p together  promote entry of Kex2p into  newly forming TGN transport vesicle at PVC.
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Figure 12: Model for role of Soi1p, TLS1, and TLS2 in the cycling of Kex2p between the TGN and PVC. TLS2 inhibits/delays entrance of Kex2p into the PVC transport vesicle at the TGN. Soi1p inhibits this function of TLS2. TLS1 and Soi1p together promote entry of Kex2p into newly forming TGN transport vesicle at PVC.

Mentions: A model for Soi1p function and the localization of TGN transmembrane proteins such as Kex2p and Ste13p is presented in Fig. 12. In this model, Soi1p promotes cycling of TGN transmembrane proteins between the TGN and PVC. Soi1p antagonizes the function of “retention” signals (TLS2 in Kex2p) in the TGN, promoting entry of proteins into transport vesicles targeted to the PVC. At the PVC, Soi1p promotes entry of proteins containing aromatic residue–containing retrieval signals (TLS1 in Kex2p) into retrograde transport vesicles. The proposed function of Soi1p at two organelles and at two distinct steps in this cycling pathway is an unusual and important feature of this model. A second important feature is that Soi1p function is not required for transport of proteins between the TGN and PVC per se. Rather, Soi1p regulates the function of TLS1 and TLS2 which, in turn, regulate the entry of proteins into the cycling pathway. The localization of forms of Kex2p that lack both signals (e.g., Y713A I718tail Kex2p or CtailΔ Kex2p) is unaffected by the loss of Soi1p. In other words, the function of Soi1p in promoting cycling is achieved through regulating the function of localization information and not through the efficiency of the transport steps that constitute this pathway. The role proposed for Soi1p is different from that of other proteins shown to be involved in TGN-to-PVC transport, including clathrin heavy chain and Vps1p. These proteins appear to be required for transport of all proteins from the TGN to the PVC. Loss of their function blocks transport of proteins regardless of the presence of signals in their tails (28, 36).


SOI1 encodes a novel, conserved protein that promotes TGN-endosomal cycling of Kex2p and other membrane proteins by modulating the function of two TGN localization signals.

Brickner JH, Fuller RS - J. Cell Biol. (1997)

Model for role of  Soi1p, TLS1, and TLS2 in the  cycling of Kex2p between the  TGN and PVC. TLS2 inhibits/delays entrance of Kex2p  into the PVC transport vesicle at the TGN. Soi1p inhibits this function of TLS2.  TLS1 and Soi1p together  promote entry of Kex2p into  newly forming TGN transport vesicle at PVC.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2139830&req=5

Figure 12: Model for role of Soi1p, TLS1, and TLS2 in the cycling of Kex2p between the TGN and PVC. TLS2 inhibits/delays entrance of Kex2p into the PVC transport vesicle at the TGN. Soi1p inhibits this function of TLS2. TLS1 and Soi1p together promote entry of Kex2p into newly forming TGN transport vesicle at PVC.
Mentions: A model for Soi1p function and the localization of TGN transmembrane proteins such as Kex2p and Ste13p is presented in Fig. 12. In this model, Soi1p promotes cycling of TGN transmembrane proteins between the TGN and PVC. Soi1p antagonizes the function of “retention” signals (TLS2 in Kex2p) in the TGN, promoting entry of proteins into transport vesicles targeted to the PVC. At the PVC, Soi1p promotes entry of proteins containing aromatic residue–containing retrieval signals (TLS1 in Kex2p) into retrograde transport vesicles. The proposed function of Soi1p at two organelles and at two distinct steps in this cycling pathway is an unusual and important feature of this model. A second important feature is that Soi1p function is not required for transport of proteins between the TGN and PVC per se. Rather, Soi1p regulates the function of TLS1 and TLS2 which, in turn, regulate the entry of proteins into the cycling pathway. The localization of forms of Kex2p that lack both signals (e.g., Y713A I718tail Kex2p or CtailΔ Kex2p) is unaffected by the loss of Soi1p. In other words, the function of Soi1p in promoting cycling is achieved through regulating the function of localization information and not through the efficiency of the transport steps that constitute this pathway. The role proposed for Soi1p is different from that of other proteins shown to be involved in TGN-to-PVC transport, including clathrin heavy chain and Vps1p. These proteins appear to be required for transport of all proteins from the TGN to the PVC. Loss of their function blocks transport of proteins regardless of the presence of signals in their tails (28, 36).

Bottom Line: Deletion of SOI1 improved retention of Tyr713Ala Kex2p in the pro-alpha-factor processing compartment but, unlike the original soi1 alleles, did not increase the half-life of Tyr713Ala Kex2p.This hypothesis was confirmed in several ways.TLS2 delayed exit of Kex2p from the TGN, whereas TLS1 did not affect this step.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor 48109-0606, USA.

ABSTRACT
Localization of yeast Kex2 protease to the TGN requires a signal (TLS1) in its cytosolic tail (C-tail). Mutation of TLS1 results in rapid transit of Kex2p to the vacuole. Isolation of suppressors of the Tyr713Ala mutation in TLS1 previously identified three SOI genes. SOI1, cloned by complementation of a sporulation defect, encodes a novel, hydrophilic 3,144-residue protein with homologues in Caenorhabditis elegans, Drosophila melanogaster, and humans. Epitope-tagged Soi1p existed in a detergent-insensitive, sedimentable form. Deletion of SOI1 impaired TGN localization of wild-type Kex2p and a fusion protein containing the C-tail of Ste13p, and also caused missorting of carboxypeptidase Y and accelerated vacuolar degradation of the Vps10p sorting receptor. Deletion of SOI1 improved retention of Tyr713Ala Kex2p in the pro-alpha-factor processing compartment but, unlike the original soi1 alleles, did not increase the half-life of Tyr713Ala Kex2p. These results suggested that Soi1p functions at two steps in the cycling of Kex2p and other proteins between the TGN and prevacuolar compartment (PVC). This hypothesis was confirmed in several ways. Soi1p was shown to be required for optimal function of TLS1. Suppression of the Tyr713Ala mutation by mutation of SOI1 was shown to be caused by activation of a second signal (TLS2) in the Kex2p C-tail. TLS2 delayed exit of Kex2p from the TGN, whereas TLS1 did not affect this step. We propose that Soi1p promotes cycling of TGN membrane proteins between the TGN and PVC by antagonizing a TGN retention signal (TLS2) and facilitating the function of a retrieval signal (TLS1) that acts at the PVC.

Show MeSH
Related in: MedlinePlus